A Nation Divided

One of the most striking things about American politics is how deep its regional political leanings, which in broad strokes have been pretty stable since the time of the Revolutionary War if not earlier, and have been static on a county by county basis since at least 1876.

Excluding the District of Columbia and single Congressional Districts, there is a 64.3 percentage point spread in Presidential preferences and an 84.9 percentage point spread in partisan U.S. Senate preferences. The extent to which Red State and Blue State America are different countries politically is very real.

This is clearly reflected in the final state by state predictions from 538:

Final FiveThirtyEight Forecast

Who’s ahead in each state and by how much

"Our win probabilities come from simulating the election 10,000 times, which produces a distribution of possible outcomes for each state. Here are the expected margins of victory [in percentage points]"

Expected margin of victory" -26 states D and 25 states R

D.C.                                  D+71.3
Vermont                            D+27.8
Maryland                          D+25.6
Hawaii                              D+24.9
Massachusetts                  D+23.3
California                         D+22.8
New York                         D+19.2
Rhode Island                    D+15.1
Maine 1st District            D+14.2
Washington                      D+13.2
Connecticut                      D+12.9
Illinois                              D+12.8
Delaware                          D+12.7
New Jersey                       D+11.7
Oregon                              D+9.2
Maine - statewide             D+7.8
New Mexico                     D+5.9
Minnesota                         D+5.7
Virginia                             D+5.5
Wisconsin                         D+5.3
Michigan                           D+4.3
Colorado                           D+4.0
Pennsylvania                     D+3.7
New Hampshire                D+3.6
Nevada                              D+1.2
Maine 2nd District            D+0.8
Florida                               D+0.6
North Carolina                  D+0.6

Ohio                                  R+1.8
Nebraska 2nd District       R+2.3
Arizona                             R+2.3
Iowa                                  R+2.7
Georgia                             R+4.2
South Carolina                  R+7.2
Alaska                              R+7.8
Texas                                 R+8.8
Missouri                            R+10.0
Utah                                  R+11.2
Indiana                              R+11.5
Tennessee                          R+12.4
Kansas                               R+13.0
Mississippi                        R+13.1
Montana                            R+14.9
Nebraska 1st District         R+15.4
South Dakota                     R+15.6
Louisiana                           R+16.3
Nebraska - statewide         R+18.1
Kentucky                           R+18.6
Idaho                                 R+20.1
Arkansas                           R+20.8
Alabama                            R+22.3
North Dakota                    R+22.7
Oklahoma                          R+26.3
West Virginia                    R+26.4
Wyoming                           R+35.1
Nebraska 3rd District        R+36.5

"Who’s ahead in each Senate race

Our win probabilities come from simulating the election 10,000 times, which produces a distribution of possible outcomes for each race.

Incumbent party Expected margin of victory"

New York            D                D+41.1
Hawaii                 D                D+41.0
Vermont               D                D+40.7
Oregon                 D                D+27.9
Maryland              D                D+27.5
Connecticut          D                D+25.5
Washington          D                 D+16.7
Illinois                  R                 D+12.3
Colorado              D                 D+8.3
Wisconsin            R                 D+3.9
Pennsylvania        R                 D+1.2
Nevada                 D                 D+1.2
New Hampshire   R                 D+0.2

Missouri               R                  R+0.7
North Carolina     R                  R+2.2
Indiana                 R                  R+2.5
Florida                  R                 R+5.5
Kentucky              R                  R+9.7
Arizona                 R                 R+10.4
Georgia                 R                 R+12.5
Ohio                      R                 R+14.8
Arkansas               R                 R+17.7
Iowa                      R                 R+18.5
Louisiana              R                 R+23.8
South Carolina      R                 R+24.4
Alaska                   R                 R+26.4
Idaho                     R                 R+29.2
Kansas                   R                 R+30.4
Alabama                R                 R+30.7
South Dakota         R                 R+32.1
Utah                       R                 R+36.5
Oklahoma              R                 R+37.4
North Dakota         R                 R+44.8

The balance of power in the next Senate

In each simulation of the Senate elections, we forecast the winner of all 34 races and note the resulting number of seats that would be held by the parties. That gives us a distribution of possible outcomes.

R  D  Change Likelihood

42 58 +12 <0.1%

43 57 +11 0.2%

44 56 +10 0.5%

45 55 +9 1.2%

46 54 +8 3.5%

47 53 +7 7.4%

48 52 +6 10.9%

49 51 +5 14.9%

50 50 +4 16.6%

51 49 +3 15.9%

52 48 +2 13.9%

53 47 +1 9.6%

54 46 —— 4.2%

55 45 -1 1.0%

56 44 -2 0.2%

In the case of 538's Senate predictions it is worth recalling that 538 systemically overweights rare possibilities in the tails of a probability distribution, so the real range of possibility is much more like 55 to 46 Democrats with a midrange value closer than indicated, but their methods are not biased, so the average result should be about the same either way (which is a strong likelihood of the modal 50-50 split with a Vice President holding the swing vote).